1. Field of the Invention
The present invention relates to a method for forming a layer of hemispherical grains and for fabricating a capacitor of a semiconductor device in the fabrication of a highly integrated semiconductor device, and more particularly to a method for forming a layer of hemispherical grains having a desired density and a desired shape in order to increase the surface area of the storage electrode of a capacitor.
2. Description of the Prior Art
The recent high integration trend of semiconductor devices inevitably involves a reduction in cell dimension. For this reason, it is necessary to form capacitors having a large capacitance while reducing topology. In the case of a dynamic random access memory (DRAM) device constituted by one transistor and one capacitor, in particular, it is important to increase the capacitance of the capacitor. In order to achieve an increase in capacitance, various methods have been proposed concerning the use of a material exhibiting a high dielectric constant, the use of a dielectric film having a small thickness and the use of a lower electrode having an increased surface area. Although various materials, such as Ta.sub.2 O.sub.5, TiO.sub.2 or SrTiO.sub.3, have been proposed as the dielectric material exhibiting a high dielectric constant, their reliance and thin film characteristics have not been confirmed. The use of a dielectric film having a reduced thickness results in the easy damage of the dielectric film occurring during an operation of the semiconductor device using the dielectric film.
In order to increase the surface area of a storage electrode, various structures have also been proposed. For example, they include a stack structure, a fin structure and a cylinder structure.
For the stack and cylinder structures, a method has also been proposed, in which a layer of hemispherical grains are formed on the surface of a storage electrode in order to increase the surface area of the storage electrode.
In this case, the shape and density of hemispherical silicon grains may vary in accordance with the condition of a silicon layer which is used as an underlayer for the growth of those hemispherical silicon grains. For this reason, it is necessary to vary the condition for the formation of such hemispherical silicon grains in accordance with the concentration of impurity ions doped in the under silicon layer and the crystalline structure of the under silicon layer. However, this is troublesome. If no variation in the process condition for the formation of hemispherical silicon grains is made when the condition for the formation of the under silicon layer varies, it is then difficult By to obtain desired density and shape of hemispherical silicon grains.